Soft bricks are 5 times the insulation of hard bricks. Hard bricks absorb the heat and use a lot more BTUs to heat the kiln.

I was looking for a site with the bendel Burner plans. I built two in 1971.
They used Grainger squirrel cage blowers. If I find the plans, I'll post them. I might have a purple ditto print of the plans in an ancient notebook.